1. Field of the Invention
The present invention relates to radio navigation generally, and more particularly to radio navigation systems in which a radio beacon radiates a signal exhibiting an artificial Doppler signal component produced by programmed variation of the radiation source location.
2. Description of the Prior Art
In British Pat. No. 1,225,190 and 1,234,541, systems are described using linear array radio beacons as applied to a Doppler microwave landing system, in which a source of radio frequency energy is commutated to separate radiator elements in order to simulate unidirectional or bidirectional constant velocity motion of the source. This produces an "artificial" Doppler signal component as observed at a remote receiving station. Navigational information is derived from these beacons by virtue of the fact that this Doppler shift of frequency is proportional to the cosine of the angle which a radio receiver of the system subtends with respect to the line of movement of the source.
The prior art Doppler navigation systems above referenced typically operate at a radio frequency of one or more GHz, and since the maximum Doppler frequency shift is of the order of a few KHz, it is highly desirable in practice to use a reference antenna element at the beacon in addition to the commutated array, the said reference antenna radiating a second radio frequency slightly offset from the basic commutated frequency, e.g., by 20 KHz. The Doppler shift to the frequency of the "moving" component is then detected as a change on the beat frequency between the received moving component signal and the received reference signal. Thus the indicated change of beat frequency, which bears the navigational information, is determined by the change of path difference between successive path lengths.
With the fixed reference antenna of the above-described system, this change of path length arises substantially only from the movement thus simulated by the commutated array, and there is the possibility of severe multipath situations occurring due to interference with the reference component path of reception being radiated from a fixed point. A multipath situation is basically one in which, in addition to the desired signal component (received by direct propagation) there are also generated unwanted components (by reflection).
Particularly with a horizontal, azimuth guidance array and at very low altitudes, a multipath situation may occur, due to signal reflection from terrain features or man-made structures. Such a situation can seriously compromise the integrity of the reference signal. If a multipath signal occurs of substantially the same strength as, and in phase opposition with the reference component from a single reference antenna, the reference component will, at least temporarily, tend to disappear at the receiving station. This results in loss of azimuth guidance at a crucial point of an aircraft's landing.
In order to avoid the worst effects of that situation and to thereby improve the performance of the beacon, it has been proposed that the two radio frequencies, ("reference" and "bearing signal"), be commutated in opposite directions along a linear antenna array, thus effectively eliminating the fixed reference antenna. That system is described in U.S. patent application Ser. No. 479,770 filed June 17, 1974, now U.S. Pat. No. 3,953,854, and assigned to the assignee of the present application. Another invention addressing the same general problem is described in U.S. Pat. application Ser. No. 574,853 filed May 6, 1975. That device provides spatial diversity for the reference component in a linear array beacon, the reference signal being moved over the baseline of the reference array in an irregular manner, the same irregular coding being superimposed on the scan program of the main array.
While the two prior approaches to the defined problem are very helpful, a still further improvement may be effected if "pencil beam" (rather than "fan beam") characteristics can be obtained from the ground beacon system.
The manner in which that improvement is effected by the arrangement of the present invention will be understood from the description hereinafter provided.